Biochips are nowadays in widespread use both in research and in industry. Essentially they comprise a generally flat solid substrate on which biomolecules are fixed, such as strands of DNA or RNA, proteins, antigens, antibodies, aptamers, etc. . . . , or indeed entire or fractioned microorganisms such as bacteria, cells, viruses, spores, etc. . . . , or indeed microobjects that themselves carry biomolecules.
Known fluorescence biochips are generally constituted by a glass slide having its surface chemically functionalized, such that after reacting with components of interest, it carries fluorescent spots that respond to light excitation at a given wavelength by emitting light at another wavelength.
The emitted light may be collected by an appropriate optical system and transmitted to a sensor, e.g. using photodetectors of the charge-coupled device (CCD) type or of the complementary metal oxide semiconductor (CMOS) type. The image that is obtained of the surface of the biochip comprises light spots of intensities that are a function of the quantity of chromophores that are present at said spots. In order to analyze these spots appropriately, it is necessary to have an image of the spots that presents sufficient fidelity and resolution.
The chromophores that are placed at the interface between a medium of higher index and a medium of lower index (typically a glass-air or glass-liquid interface) emits preferentially into the higher index medium at a ratio that is equal (to a first approximation) to the cube of the ratio of the refractive indices of the two media, i.e. about 4.1 for glass and air, which means that when the sensor is placed above the biochip, about 80% of the light emission from the chromophores is lost in the glass slide.
In addition, the optical systems used present a numerical aperture that is limited such that only a small fraction of the light emitted into the air can be transmitted to the sensor. The overall efficiency with which the light emitted by the chromophores is collected is then restricted to a few percent.
In order to reduce those drawbacks, proposals have already been made to capture the light the chromophores emit into the higher-index medium. In this context, the simplest implementation consists in using the sensor itself as the substrate of the biochip, and thus in placing an array of biological probes on the surface of the sensor, which probes are subsequently put into contact with the molecules for analysis.
The drawback of that technique is its cost, since each analysis requires a new sensor to be used.